1. Field of the Invention
The present invention relates generally to the field of packaging materials, particularly packaging materials for use with moisture vapor and oxygen sensitive products. It concerns improving moisture vapor and oxygen barrier properties of polyethylene resins by blending them with relatively small amounts of low molecular weight hydrogenated aliphatic hydrocarbon resins.
2. Description of Related Art
Polyethylenes can be classified into two types: (i) high pressure branched low density polyethylene or LDPE and (ii) low pressure or linear polyethylene. Low pressure or linear polyethylene can be further separated into three categories: (iia) high density polyethylene or HDPE, (iib) linear low density polyethylene or LLDPE and (iic) ultra linear low density polyethylene or ULLDPE. Branched LDPE was the first type of polyethylene to be commercialized, and it can be polymerized in high pressure tubular or autoclave reactors. The density of branched LDPE is typically between about 0.910-0.940 g/cm3; it can be heavily and randomly branched with a significant fraction of its branches being longer than the critical entanglement molecular weight of polyethylene.
Polyethylenes with densities higher than about 0.940 g/cm3 are typically referred to as HDPE. These are produced with no comonomer or with very small amounts of comonomers such as xcex1-olefins like hexene, butene, and octane, among others. Comonomer is mixed in with ethylene at low pressures during the polymerization process. HDPE can be produced in slurry loop reactors, solution form reactors or in gas phase fluidized bed reactors. Certain types of HDPE polymer have a small amount of long chain branching.
Linear LDPEs (LLDPEs) are obtained by incorporating sufficient xcex1-olefin comonomers into linear polyethylene to produce polyethylene with a density between about 0.910-0.940 g/cm3. The xcex1-olefin comonomers are essentially excluded from the crystal lattice of polyethylene; therefore, their presence serves to disrupt the crystallizability of the linear chain, which can lead to polyethylenes having lower densities (crystallinity). By definition, LLDPEs comprise essentially no (e.g., less than about 0.1% by weight of the polyethylene) long branches (e.g., branches longer than the critical entanglement molecular weight of polyethylene). These polymers can be produced in low pressure gas phase fluidized bed reactors, solution process reactors or in slurry loop reactors.
Ultra linear low density polyethylenes, ULLDPEs, which incorporate even higher levels of xcex1-olefin comonomers into linear polyethylene have densities lower than about 0.910 g/cm3 and can be produced in reactors similar to those used to produce LLDPEs.
Polyethylenes can be used in the manufacture of a number of different packaging items using a variety of conversion processes such as blow molding, injection molding, sheet extrusion, blown film extrusion and cast film extrusion. Polyethylenes can also be used as components in multilayer packaging articles, which are manufactured using coextruded blown film, coextruded cast film, coextruded blow molding and other processes such as extrusion coating and laminations. Through the use of these various processing techniques, polyethylenes can be used to package a number of different food and non-food finished goods such as milk (e.g., blow molded bottles), bread (e.g., blown and cast films), paper products (e.g., extrusion coating, cast and blown film), applesauce (thermoformed sheet) and cleaning supplies (e.g., injection and blow molded bottles).
Polyethylenes are used in packaging items because they provide at least some resistance to moisture, gases, acids, bases, and solvents, while retaining package integrity by virtue of their impact strength, tear strength, stiffness, and other key attributes or properties. While certain polyethylenes have relatively low moisture vapor and oxygen transmission rates, there are other polymers with still lower moisture and oxygen transmission rates than polyethylenes. Thus, it would be advantageous to produce packaging items comprising polyethylene (e.g., HDPE, LLDPE, ULLDPE and LDPE) with improved moisture and oxygen barrier, while minimizing any increase in manufacturing cost or significantly decreasing other desirable properties imparted by polyethylene to the packaging item.
One aspect of the present invention is directed to compositions comprising relatively low levels (e.g., less than 3 wt %) of a low molecular weight hydrogenated aliphatic resin and a linear high density polyethylene (HDPE). Preferably such a composition comprises between about 99.5 wt % and 95 wt % of a high density polyethylene. The composition can have a normalized moisture vapor transmission rate (normalized MVTR) of less than about 0.35 gxc2x7mil/100 in2/day, when a blown film is prepared from the composition. By convention, the value obtained for MVTR is expressed as grams of water transmitted per 100 square inches in a 24-hour period (or, in metric system, grams of water transmitted per square meter in a 24-hour period). This value can be normalized for film thickness by several commonly accepted methods. The MVTR values referenced in this application are normalized for film thickness by multiplying the MVTR g/100 in2/day by the film thickness in mils resulting in a unit of gxc2x7mil/100 in2/day (e.g., normalized MVTR). Such compositions of the present invention can be incorporated in packaging articles, particularly packaging articles used in packaging moisture vapor sensitive products, thereby reducing the exposure of the packaged product to moisture vapors and/or oxygen.
Another aspect of the present invention is directed to a method of preparing such compositions by blending HDPE and less than 3 wt % of a low molecular weight hydrogenated aliphatic resin. In certain embodiments the blend can be extruded, coextruded with other resins, or the blend can be otherwise incorporated into the structure of a packaging article.
Still another aspect of the invention is directed to a packaging article comprising at least one layer that comprises less than 3 wt % of a low molecular weight hydrogenated aliphatic resin and between about 99.5 wt % and 95 wt % high density polyethylene. Such packaging articles can be coatings, molded articles, or films, and the films can be cast, blown or undergo other post oriented processing.
Yet another embodiment of the present invention is directed to a moisture resistant composition comprising a low molecular weight hydrogenated aliphatic resin that has a weight-average molecular weight of less than about 2000 g/mol, more preferably between about 50 g/mol and 2000 g/mol, most preferably between about 50 g/mol and 1000 g/mol, a low molecular weight high density polyethylene and a second high density polyethylene. The low molecular weight high density polyethylene has a zero-shear viscosity that is less than or equal to about 0.9 times the zero-shear viscosity of the second high density polyethylene, and the second high density polyethylene preferably has a melt index of between about 0.1 dg/min and 100 dg/min.
The composition comprises between about 0.5 and 25 wt % low molecular weight hydrogenated aliphatic resin, between about 8 and 30 wt % low molecular weight high density polyethylene, and between about 45 and 92.5 wt % of the second high density polyethylene. Preferably, the second high density polyethylene is essentially the only other component in the composition besides the low molecular weight hydrogenated aliphatic resin and the low molecular weight high density polyethylene.
A further embodiment of the present invention is directed to a moisture resistant composition, comprising between about 0.5 and 4 wt % of a low molecular weight hydrogenated aliphatic resin, between about 1 and 30 wt % of a low molecular weight high density polyethylene, and between about 66 and 98.5 wt % of a second high density polyethylene. As in embodiments described above, the low molecular weight resin has a weight-average molecular weight of less than about 2000 g/mol, and the low molecular weight high density polyethylene has a zero-shear viscosity that is less than or equal to 0.9 times the zero-shear viscosity of the second high density polyethylene. The second high density polyethylene can have a melt index less of between about 0.1 dg/min and 100 dg/min. Preferably, the composition comprises between about 0.5 and 3.5 wt % of the low molecular weight hydrogenated aliphatic resin, and the second high density polyethylene is essentially the only other component in the composition besides the low molecular weight hydrogenated aliphatic resin and the low molecular weight high density polyethylene.
Certain compositions of the present invention comprise less than about 25 wt % low molecular weight hydrogenated aliphatic resin and a HDPE. Addition of the hydrogenated aliphatic resin at low levels, particularly less than about 3 wt %, to HDPE can permit the blend to have physical properties similar to that of HDPE, while decreasing the moisture vapor transmission rate (MVTR) of the composition relative to that of the base HDPE. That is, certain compositions of the present invention have stiffness and tear resistance similar to that of HDPE blown films, while having a lower MVTR than HDPE, and in certain cases the oxygen transmission rates (OTRs) of the compositions are also less than that of HDPE alone. In addition, limiting the concentration of the low molecular weight hydrogenated aliphatic resin to less than about 3 wt % reduces the amount of resin which can migrate from the packaging article to the food stuffs which are being packaged. Furthermore, since such low molecular weight hydrogenated aliphatic resins can be relatively expensive, certain compositions of the present invention having relatively low percentages of these resins could be produced without substantially increasing the price over that of bulk HDPE.
Another embodiment of the present invention is directed to a moisture resistant composition that comprises between about 0.5 and 25 wt % of a low molecular weight hydrogenated aliphatic resin and between about 99.5 wt % and 75 wt % of a branched or linear low density polyethylene (e.g., branched low density polyethylene (LDPE), linear low density polyethylene (LLDPE), or ultra linear low density polyethylene (ULLDPE)). The branched or linear low density polyethylene has a density of less than about 0.940 g/cm3.
As with the compositions comprising HDPE described above, certain compositions of the present invention comprising low molecular weight hydrogenated aliphatic resin and a branched or linear low density polyethylene can have physical properties similar to that of the branched or linear low density polyethylene, while decreasing the moisture vapor transmission rate (MVTR) of the composition relative to that of the base branched or linear low density polyethylene. That is, certain compositions of the present invention that comprise a branched or linear low density polyethylene and a low molecular weight hydrogenated aliphatic resin can have stiffness and tear resistance similar to that of the branched or linear low density polyethylene, while having a lower MVTR than the branched or linear low density polyethylene, and in certain cases the oxygen transmission rates (OTRs) of the compositions are also less than that of the branched or linear low density polyethylene alone.
Hydrogenated aliphatic resins can have good compatibility for blending with polyethylene. Compositions of the present invention used in packaging can have fewer negative organoleptic effects on packaged products than polyethylene blends comprising low molecular weight hydrocarbon resins that are not hydrogenated. For example, compositions of the present invention have characteristics that should allow them to be approved for food packaging uses. Furthermore certain compositions of the present invention can be processed as films, and down-gauging of such films can result in thinner films that have similar MVTR performance as the base polyethylene.